1. Field of the Invention
The invention relates to a seismic prospection method providing better knowledge of the position and of the nature of the geological discontinuities of the subsoil.
2. Description of the Prior Art
Seismic prospection methods generally include a step for transmitting seismic waves, a step for receiving at one or more positions waves which are propagated in the ground to be studies, a step for recording the signals received and a step for applying to the recorded signals processing operations of very different kinds before drawing up seismic sections in which the discontinuities of the subsoil are restored in their true place.
A known seismic prospection method includes the transmission of seismic waves from a source disposed either at the surface of the ground or in another well and the reception of the waves after propagation at one or more positions in a well bored in the ground.
The reception is provided by means of a reception device having at least one probe of small section, suspended by an electric suspension cable from a support assembly disposed on the surface, and having retractable anchorage arms for applying the body of the probe against the wall of the well. In a compartment of the probe body are disposed one or more directional seismic sensors.
These sensors are generally associated in threes and their axes disposed along three orthogonal axes.
The reception device may include also several well probes suspended below each other and each containing at least one triaxial sensor, so as to increase the positions for receiving the seismic waves. The probe or the probe assembly is lowered into the well and raised in successive steps. At each pause, a cycle of seismic wave transmission is carried out.
The waves which are propagated as far as the probe or the probe assembly are received by the triaxial sensors and the seismic signals are collected by an acquisition assembly which transmits them to a surface laboratory.
Such well probes are described for example in the French Pat. Nos. 2 501 380 and 2 548 727 and the French patent applications EN 87/04365 and 87/04677.
The signals received are generally complex. Geophysicists have sought to determine the curve described by the center of gravity of each triaxial receiver and designated hereafter by trajectory, from the signals received by the three elementary sensors of the same receiver and have discovered that the surface which was best able to represent it, at least for a certain time from the beginning of reception, was an ellipsoid generally very flattened.
It has been considered up to now that this flattened ellipsoid modeling the real trajectory was contained in a substantially vertical plane, the deviations being neglected, and that this vertical plane was the one which passed through the seismic source and the position of each receiver.
Since the probe or each of them is able to rotate to a greater or lesser degree about its axis during the lowering or raising operations, the trihedron formed of the respective axes of the three elementary sensors of each receiver may have any orientation with respect to the vertical plane passing through the source and the reception position. On this assumption, it was neessary to compare and combine the signals delivered by the sensors of the same receiver so as to determine the dihedral angle of the vertical plane of the trajectory with a vertical reference plane formed by two axes of the trihedron and the angle between the axes of symmetry of the trajectory and the horizontal plane. These two angles which, by analogy may be designated by longitude and latitude, were sufficient for characterizing the position of the trajectory. The knowledge of these two angles and of other parameters such as the ellipticity of the trajectory made it possible, on this assumption, to determine certain geophysical and geological characteristics of the ground passed through by the waves.
The seismic prospection method of the invention improves very appreciably the knowledge of the geological characteristics of the ground passed through.